<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, minimum-scale=1, user-scalable=no">
    <title>2.4 单指拖拽--拖拽</title>
    <style>
        body {
            height: 2000px;
        }
        .backtop {
            position: fixed;
            right: 20px;
            bottom: 20px;
            width: 45px;
            height: 45px;
            line-height: 45px;
            text-align: center;
            background-color: rgba(0, 0, 0, 0.6);
            border-radius: 50%;
            color: #fff;
            font-size: 30px;
            -webkit-tap-highlight-color: transparent;
            /*transform: translate3d(x, y, 0);*/
        }
    </style>
</head>
<body>
    <a href="#" id="backtop" class="backtop">&uarr;</a>

    <script>
        function drag(el, options) {
            options.x = typeof options.x !== 'undefined' ? options.x : true;
            options.y = typeof options.y !== 'undefined' ? options.y : false;

            if (!options.x && !options.y) return;

            var curPoint = {
                x: 0,
                y: 0
            };
            var startPoint = {};
            var isTouchMove = false;

            el.addEventListener('touchstart', handleStart, false);
            el.addEventListener('touchmove', handleMove, false);
            el.addEventListener('touchend', handleEnd, false);

            function handleStart(ev) {
                var touch = ev.changedTouches[0];

                startPoint.x = touch.pageX;
                startPoint.y = touch.pageY;
            }
            function handleMove(ev) {
                ev.preventDefault();

                isTouchMove = true;

                var touch = ev.changedTouches[0];
                var diffPoint = {};
                var movePoint = {
                    x: 0,
                    y: 0
                };

                diffPoint.x = touch.pageX - startPoint.x;
                diffPoint.y = touch.pageY - startPoint.y;

                if (options.x) {
                    movePoint.x = diffPoint.x + curPoint.x;
                }
                if (options.y) {
                    movePoint.y = diffPoint.y + curPoint.y;
                }

                move(el, movePoint.x, movePoint.y);
            }
            function handleEnd(ev) {
                if (!isTouchMove) return;

                var touch = ev.changedTouches[0];

                curPoint.x += touch.pageX - startPoint.x;
                curPoint.y += touch.pageY - startPoint.y;

                isTouchMove = false;
            }

            function move(el, x, y) {
                x = x || 0;
                y = y || 0;

                el.style.transform = 'translate3d(' + x + 'px, ' + y + 'px, 0)';
            }
        }
    </script>
    <script>
        var backtop = document.getElementById('backtop');
        drag(backtop, {
            x: true,
            y: true
            // y: true
        });
    </script>
</body>
</html>